1. Field of the invention
This invention relates generally to a washing machine including an electric motor for driving a rotatable tub and an agitator, and more particularly to control of a speed increase rate of the motor in such a washing machine.
2. Description of the prior art
Conventional washing machines comprises a single-phase induction motor of the capacitor type for driving an agitator and a rotatable tub serving both as a wash tub and a dehydration tub. Voltage of a commercial AC power-supply is applied to the motor when the same is energized. Accordingly, a rotational speed of the motor is rapidly increased toward one according to a frequency of the commercial AC power-supply and the number of poles of the motor.
The rotatable tub swings to a large extent being rotated or vibrates to thereby produce noise in a dehydration operation when laundry is ununiformly distributed in the rotatable tub. To overcome this problem, the prior art has provided a washing machine in which the motor speed is switched in a plurality of stages by tap changing or phase control so that the motor speed is increased stepwise. In these control manners, however, the motor speed is increased still rapidly stepwise such that the motor produces electromagnetic noise or the rotatable tub vibrates. Consequently, a desired noise reduction cannot be achieved.